1. Field of the Invention
This invention relates to a method of identifying the denominations of pieces of paper, particularly to a method of identifying the denominations of bills inserted into machines such as automatic vending machines and automatic money exchanging machines.
2. Prior Art
Machines such as automatic vending machines and automatic money changing machines for dispensing merchandise or coins are provided with a device for identifying a denomination of an inserted bill.
In such machines, as shown in FIG. 5, a bill 1 is transferred by a conveying means such as a belt, the image data of the print pattern of the bill 1 is read by a photo sensor 2 disposed at predetermined positions with respect to the conveying means as the bill 1 advances a predetermined distance, and the image data is compared with the data of the print patterns stored in memory 3 to determine the denomination of the bill 1.
The photo sensor 2 comprises a light emitting part 4 and a light receiving part 5. The light emitting part 4 emits a constant quantity of visible light using an LED as the light source. The light emitted from the light emitting part 4 is irradiated onto the bill 1 and passes through it. Since the quantity of transmitting light varies according to the color and the density of the print pattern of the bill 1, the print pattern is detected by receiving the transmitted light and converting it into an electrical signal corresponding to the quantity of light with the light receiving part 5.
The image data of all denominations of bills, which is utilized to identify the bills, is stored in memory 3, and the image data of each bill is looked up and compared with the image data detected with the photo sensor 3 by a pattern matching technique. Since a bill may be conveyed to the photo sensor 2 forward or backward with its right or reverse side, generally four sets of image data are stored in the memory 3 for each denomination of bills. If a machine can accept six denominations of bills, pattern matching is performed 24 times when a bill is inserted.
To make it easier to understand, the conventional identifying method is described below in more detail, taking for example a case in which only one set of image data is stored in memory for each denomination of bill (on the assumption that bills are always conveyed in their predetermined one orientation) and six denominations of bills are identified.
FIG. 6 illustrates the concept of the conventional method identifying one of six denominations of bills. In FIG. 6, six ellipses A to F indicate the distribution of the image data obtained by reading six kinds of denomination of bills with the photo sensor, respectively. The mean point of each distribution is shown by a black point.
The distributions include errors caused by variation in reading a bill, in transferring a bill, and by creases and wrinkles of a bill. A space shown in FIG. 6 is an n-dimensional Euclidean space for representing a set of the values of n inputs of data used for identification as a point in it, but it is shown as a 2-dimensional plane for convenience in this specification.
Now suppose that the image data of an unknown bill read by the photo sensor is represented by point X in FIG. 6. Then the distances between point X and the mean point of the distribution of the image data of each denomination (degree of similarity) are compared with each other, and the denomination of the mean point of which is at the smallest distance (highest similarity), D in FIG. 6, is determined to be the denomination of the bill.
Actually, this verification of similarity is performed by calculating inner products of the normalized vector of the mean points and the normalized vector of point X, and then comparing the values of the inner products. This method is generally called "simple similarity comparison" and widely used. There is another method called "combined similarity comparison" that further uses the characteristic vector for each denomination for the calculation of the similarity.
However, there is a problem with the methods described above because their capability is not sufficient to identify bills when their outside dimensions are the same and the difference in the design of denominations is very small, such as US dollar bills.
For such bills, the distributions of the image data of all denominations are in close vicinity to each other as shown in FIG. 7, and image data X read by the photo sensor can be slightly nearer to the mean point of the distribution of denomination F than to that of denomination D though image data X is within the distribution range of denomination D, causing an incorrect determination that the similarity to denomination F is highest.
For this reason, the inserted bill must be returned to the customer as not identifiable when the difference between the similarity to a certain denomination and that to any other denomination are not sufficiently large.
This invention is made to solve the above problem, and the object of this invention is to provide a method of identifying the denominations of pieces of paper that has an improved identifying capability making it possible to correctly identify many denominations of bills, securities and the like with similar print patterns at an increased speed.